The Matsumoto-Ohtsuka (MO)-type vacuum flange, which can provide a gapless connection and a highly. reliable electric contact between flanges, was studied experimentally for a possible application to beam ducts for high-current accelerators, where the apertures have a complicated structure, such as the combination of a beam channel and one or two flat rectangular antechambers. In spite of the complex aperture, test flanges showed a good vacuum-sealing property. Vacuum sealing was successfully achieved with a reasonable fastening torque (15-20 N m), and remained sealed after baking at 250 degrees C for 24 h. A twist between flanges,of up to about 5 mrad was also manageable. A pseudoelastic structural analysis well reproduced the observed deformation of the flange. This analysis should be available for optimizing the flange structure. The MO-type vacuum flange was found to be promising for connecting flanges of the beam duct with a complicated aperture in future accelerators. The flange will also be available as a compact vacuum flange in cases where a complex aperture is required. (c) 2005 American Vacuum Society.